Re: [PATCH v19 03/14] KVM: arm64: Manage GCS access and registers for guests

On Wed, Aug 19, 2026 at 05:46:00PM +0100, Mark Brown wrote:

On Wed, Aug 19, 2026 at 05:32:25PM +0100, Leonardo Bras wrote:

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On Wed, Aug 12, 2026 at 08:12:02PM +0100, Mark Brown wrote:

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In order to allow guests to use GCS we also need to configure
HCRX_EL2.GCSEn, if this is not set GCS instructions will be noops and
CHKFEAT will report GCS as disabled.

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It is zero on reset, and keeping it in zero disables GCS in EL0&EL1, so 
unless we are in EL2&0 (HCR_EL2.{E2H, TGE} is {1, 1}), we need to enable it 
so EL1&0 (guests) can have access to it.

Right.

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@@ -77,6 +80,8 @@ static void __sysreg_save_vel2_state(struct kvm_vcpu *vcpu)
 	__vcpu_assign_sys_reg(vcpu, SP_EL2,	 read_sysreg(sp_el1));
 	__vcpu_assign_sys_reg(vcpu, ELR_EL2,	 read_sysreg_el1(SYS_ELR));
 	__vcpu_assign_sys_reg(vcpu, SPSR_EL2,	 read_sysreg_el1(SYS_SPSR));
+	if (ctxt_has_gcs(&vcpu->arch.ctxt))
+		__vcpu_assign_sys_reg(vcpu, GCSPR_EL2, read_sysreg_el1(SYS_GCSPR));

quoted

Out of curiosity here,
Why does the saving of GCSCR_EL2 depends on ctxt_has_tcrx() (and E2H set), 
and saving SYS_GCSPR does not? ... 

This is just because there isn't a preexisting ctxt_has_tcrx() check
already there, FEAT_GCS architecturally depends on FEAT_TCRX and there
was a request to make this explicit in the code to try to optimise
things a bit.  The compiler should skip over both blocks at once if TCRX
isn't there rather than having two separate tests or static branches.  I
didn't add new checks where there were none since I expect that to be
unhelpful for code generation, you'd get the reverse situation and emit
two checks.

Humm, but then why saving GCSPR_EL2 does not depend on TCRX/E2H? Or maybe a 
better question, why are not GCSPR_EL2 and GCSCR_EL2 saved in the same 'if' 
clause under ctxt_has_tcrx() (and E2H set), if they are restored in the 
same 'if' clause?

As you mentioned, GCS depends on TCRX, so it should be fine, but just by 
reading the code I see:
- GCSCR_EL2 :
	Save if 	GCS=1, TCRX=1, E2H=1
	Restore if 	GCS=1, TCRX=1
- GCSPR_EL2 :
	Save if 	GCS=1,
	Restore if	GCS=1, TCRX=1

Which looks kind of confusing for the as a first time reader.

Does it make sense?

Thanks!
Leo